Fig. 1. Optical layout of beamline BL25SU. spherical mirror grating exit slit spherical mirror cylindrical mirror twin helical cylindrical mirror 176 ° cylindrical mirror spherical mirror 177 ° undulator 177 ° 177 ° 175 ° 176 ° 178 ° 174 ° VLSPG S 1 M v M h M 1 M 2 S 2 ST 2 ST 1 ST 3 M 3 M 4 79910 76410 71910 10000 955 10955 61910 60955 61338 50000 40000 10000 2000 38000 0 entrance slit 80 HIGH RESOLUTION HELICAL UNDULATOR SOFT X-RAY BEAMLINE T h e b e a m l i n e B L 2 5 S U f o r " S o f t X - r a y Spectroscopy of Solid", was the first soft X-ray beamline commissioned at SPring-8. It was designed for high-precision studies of electronic states and surface structures of solids. The goal on the first stage operation was to p e r f o r m h i g h - r e s o l u t i o n p h o t o e m i s s i o n spectroscopy in the photon energy (h ν ) range between 0.5 and 1 keV, within which lie the L and M edges of 3 d and 4 f elements. Satisfactory performance was achieved in the spring of 1999. A schematic layout of the beamline is shown in Fig. 1. A detailed description of the beamline has been reported in a recent article [1]. The light source is a "twin helical undulator" [2] . Two helical undulators were installed in tandem in a 4.5 m straight section of the storage ring. The gap of each undulator, consisting of 12 periods of 12 cm length, can be closed independently to a minimum gap of 30 mm. Fundamental radiation can be thus emitted throughout the photon energy range between 0.22 and 3 keV. The optical system consists of pre-focusing mirrors (M h and M v ), a grating monochromator (S 1 − M 1 or M 2 − VLSPG − S 2 ) and the post-focusing mirrors ( M 3 and M 4 ). The constant deviation type monochromator employs a spherical mirror ( M 1 or M 2 ) and the varied line-spacing plane gratings (VLSPG) as monochromatizing optics. Energy resolution (E/ ∆ E) of about 10 4 or more was expected by ray tracing, even when possible slope errors (1 μ rad for the spherical mirror M 1 or M 2 and 0.5 μ rad for the plane grating) were considered [1]. The energy resolution on this beamline was experimentally tested by examining the core-level photoabsorption spectroscopy of several gases obtained by recording photo-ionized ion currents. The Ne 1 s spectrum is shown in Fig. 2. The entrance and exit slits were set to 30 μ m and 15 μ m, respectively. The 1 s - 3 p transition at 867.12 eV has a FWHM of 265 ± 3 meV. This value is slightly better than the former best value of 268 ± 5 meV obtained at ELETTRA [3]. The instrumental resolution is estimated from the least-squares fit by assuming that an observed spectral peak shape is the result of the Lorentzian natural line function ( F W H M : Γ L ) c o n v o l u t e d b y t h e G a u s s i a n monochromator resolution (FWHM: Γ G ). The fitted curve in Fig. 2 is obtained with the optimized parameters of Γ L = 246 meV and Γ G = 62 meV. Both of the two values are smaller than those r e p o r t e d t h u s f a r , i l l u s t r a t i n g t h e s u p e r i o r p e r f o r m a n c e o f t h i s m o n o c h r o m a t o r w h e n 0.6 0.4 0.2 0.0 -0.2 -0.4 Binding Energy (eV) Au Γ t = 80 meV Photoemission Intensity (arb. units) Fig. 3. Photoemission spectrum near the Fermi energy of Au. Fig. 2. 1s photoabsorption spectrum of Ne gas. 81 Yuji Saitoh a , Akira Sekiyama b and Shigemasa Suga b (a) SPring-8 / JAERI (b) Osaka University E-mail: ysaitoh@sping8.or.jp References [1] Y. Saitoh et al. , J. Synchrotron Rad. 5 (1998) 542. [2] T. Hara et al. , J. Synchrotron Rad. 5 (1998) 426. [3] M. Coreno et al. , Phys. Rev. A 59 (1999) 2494. c o m p a r e d t o i t s c o m p e t i t o r s . T h e h i g h e s t resolution to date was attained by second-order diffraction from a grating with 1200 lines/mm groove density [3], while our data was obtained by first-order diffraction from a grating with 600 lines/mm groove. This increased diffraction efficiency allows for a higher flux at experimental stations. To further analyze the monochromator resolution, we measured a photoemission spectrum near the Fermi-edge of an evaporated Au film at h ν = 867.6 eV using a GAMMADATA-SCIENTA SES-200 analyzer. Total system resolution ( Γ t ) of 80 ± 5 meV was obtained following fitting analysis (Fig. 3). The energy resolution of the electron energy analyzer using the same entrance slit and pass energy was found to be 58 meV upon measurement with He I (h ν = 21.2 eV) radiation. This spectral analysis indicates an estimated monochromator resolution ( Γ G ) of 52 ± 10 meV. We can conclude, therefore, that resolving power (E/ ∆ E) about 15,000 is achieved at ∼ 867 eV in our monochromator. We have also obtained high-resolution spectra in the lower h ν range such as the K absorption of N 2 ( ∼ 400 eV) and O 2 ( ∼ 540 eV). These spectra were found to be of equal or higher resolution than any formerly published results. Total Yield (arb. units) Ne 1 s Photon Energy (eV) 3 4 5 6 p 865 866 867 868 869 870 High stability of all optical components, an essential condition for high-resolution functioning of the monochromator, was maintained due to the low heat load from the helical undulators. The typical value of the photon flux was about 1 × 10 11 photons/sec at 870 eV with the resolution of 10 4 in the first harmonic for the upstream undulator radiation. Due to the consistency and quality of these preliminary performance assessments, various experiments of high-resolution bulk sensitive photoemission as well as core absorption magnetic circular dichroism are being performed intensively on this beamline.
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